DESCRIPTION (Applicant's Description) Mutagenesis is a precursor to cancer. Shuttle vector systems have successfully demonstrated that specific DNA lesions resulting from ultraviolet radiation (i.e. sunlight) can produce mutations important to cancer induction in skin cells. Ionizing radiation exposure is a well known cause of breast cancer, yet similar shuttle vector studies with ionizing radiation and mammary cells have not been done. Additionally, the mutagenic potential of DNA damage in cells is known to be modulated by environment factors, such physiological stress and drugs. Yet, the potential of hormones (the major modulators of mammary cell metabolism) to modify mutagenesis in mammary cells is not known. Animal studies suggest that hormones can influence radiation mutagenesis, since the timing of irradiation during the estrous cycle or during pregnancy is a significant determinant of incidence of radiation-induced breast cancer in rats. This is a application to assess the influence of hormones on ionizing-radiation-induced mutagenesis in mammary cells. Our approach will be to measure mutagenesis in hormonally-treated and/or radiation- treated mammary cells in vitro, using shuttle vectors. Shuttle vector systems are versatile in that they can be used to measure either the mutagenic potential of DNA lesions in different types of cells, or mutations in the same cells under different environmental conditions (eg. hormone- or radiation-treated). DNA will be damaged in vitro with radiation or radiomimetic drugs and then transfected into mammary cell hosts, where the damaged DNA will be metabolically processed by cellular DNA repair enzymes. The DNA will then be recovered and assayed for mutations. Since DNA synthesis, repair, and regulatory enzymes are known to be affected by physiological states, our hypothesis is that hormonal treatment will alter both the quantity and type of mutations produced by radiation. This information may suggest DNA repair mechanisms or transduction pathways that are hormonally responsive and may suggest ways of inhibiting or preventing radiation-induced breast cancer. Recent results, from our laboratory, suggest that radiation mutagenesis may occur via two major "error-prone" DNA repair pathways of mammalian cells that can be independently probed using shuttle vector plasmid DNA with either single- or double-strand breaks. We will use such plasmids to determine the potential roles of these pathways in mammary cell mutagenesis. The proposed study represents an exploratory pilot project that will hopefully develop into a future, more comprehensive, breast cancer research project. It directly addresses problems of breast cancer etiology, which is a priority area of the RFA. Under this area, it also incorporates three research topics that are of expressed interest to the RFA: effects of radiation, hormones, and gene- environment interactions. Also, in accordance with the goals of the RFA, the Principal Investigator, although well established, is new to the field of breast cancer research.